[RCU] Relay access denied

Hello all!

If I define $rcmail_config['smtp_server'] = '78.47.3.18'; instead of $rcmail_config['smtp_server'] = ''; the users get a Relay access denied.

Whereby Roundcube takes the first IP of the subnet 78.47.3.17 and I assume, this is the reason for the denied access.

Dec 25 15:14:47 delta anup/smtpd[32664]: connect from sites.anup.de[78.47.3.17] Dec 25 15:14:47 delta anup/smtpd[32664]: NOQUEUE: reject: RCPT from sites.anup.de[78.47.3.17]: 554 5.7.1 anmeyer@gmx.net: Relay access denied; from=anmeyer@anup.de to=<anmeyer@gmx.net Dec 25 15:14:47 delta anup/smtpd[32664]: disconnect from sites.anup.de[78.47.3.17]

How do I prevent this?

Andreas